Mixed Lubrication Regime

Lubrication is used not only in a large number of industrial applications, where the lubricating fluid is generally oil, but also in the biomedical field, where all our joints are lubricated by synovial fluid. In general, for most of these applications, the low levels of wear and friction are achieved by the total absence of contact due to the presence of a film completely separating the two opposing parts. However, in certain special cases, this continuous film between the two parts cannot be maintained, and direct interactions between the two parts then take place. The morphology of the fluid film must be analyzed on a microgeometric scale.

Even if it has a certain capacity for deformation, the microgeometry of the two parts does not ensure the absence of interpenetrations in the contact zone. This leads to a breakdown of the oil film at the most protruding asperities of either part, with the hollows remaining filled by the oil film. Discontinuity of the oil film is the main feature of mixed-regime lubrication. The contact is thus the site of two types of behavior, one similar to a continuous thin film and the other close to direct contact at the highest asperities.

After a first part designed to present the generalities of mixed lubrication (its positioning in the context of lubrication, followed by a phenomenological description), this article will detail the practical aspects linked to the use of mixed lubrication (influence of parameters, use cases and choice of materials). The last part of the paper will present the various methods used to study mixed lubrication. First, experimental techniques will be detailed. We conclude with a review of the various methods for modeling behavior in the presence of mixed lubrication.